The present invention relates generally to sensing of fuel level within a fuel tank. More particularly, the invention relates to an electronic circuit for minimizing corrosion of the immersed fuel sensing components by minimizing the effects of electrolysis.
In a vehicular fuel level sensing system there is a float within the fuel tank which is mechanically coupled to a fuel level sender that provides an electronic signal or condition indicative of fuel level. Typically the fuel level sender is a variable impedance device, such as a rheostat, which changes impedance as the level of the float rises and falls. In a simple system, the fuel level sender may be coupled to a fuel gage located in the passenger compartment. The current flowing through the sender is displayed on the gage to give a reading of the fuel level. In more sophisticated systems an onboard vehicle computer, sometimes called the body computer, is connected to the sender to receive data indicative of fuel level from the sender. The body computer may be programmed to provide output signals to a display in the passenger compartment for indicating fuel level as well as to provide information about fuel level for other vehicle operating purposes.
The fuel level sender of conventional design is typically in the form of an inked-on resistive trace or wire wound element placed on or secured to a plastic or other nonconductive substrate. Commonly, the sender includes silver electrodes by which the sender is electrically connected to the vehicle's electrical system and to the body computer or fuel gage. A movable wiper, also typically having a silver contact, makes sliding contact with the resistive trace in a fashion conventional to rheostats and potentiometers. The sender is typically disposed within the fuel tank in such a manner that changes in fuel level, acting through the float, change the position of the wiper with respect to the resistive trace, thereby selecting a given resistance condition which can be related to fuel level.
Because the fuel level sender is disposed inside the fuel tank, it comes into contact with the fuel stored in the tank. When using conventional gasoline fuels, the placement of the sender within the tank has not posed problems and indeed has proved beneficial since the sender components can be economically manufactured as part of the float assembly without complex mechanical linkage through the tank wall. Although suitable for conventional gasoline fuels, the above-described arrangement has proven to be quite unworkable when fuels containing a high proportion of methanol are introduced into the tank. These so-called "flexible fuels" are being developed and are under consideration to reduce hydrocarbon emissions and to reduce the consumption of petroleum-based fuels.
Many of the flexible fuels, particularly those containing methanol in high proportion, are extremely corrosive, because of the ability to absorb and mix with water. These fuels permit or faciliate electrolysis to occur at a rapid rate whereby the silver electrode material oxidizes causing the sender to fail. For example, using M85 fuel, a mixture of approximately 85% methyl alcohol and 15% gasoline. In such a u mixture, with 1/2% water added, the conventional fuel level sender has been seen to fail in approximately 72 hours of operation.
Prior solutions to the problem have employed circuits for applying very short duty cycle, single polarity pulses to the sender to minimize the percentage of time that current flows through the sender and to thereby minimize corrosion due to electrolysis. Current flow through the sender is sampled only during the brief pulse intervals when the short duty cycle signal is in the ON state.
While the variable duty cycle techniques can reduce corrosion due to electrolysis, we have discovered a different technique which appears to eliminate the effect of electrolysis action. In accordance with our invention a drive circuit is coupled to the fuel level sender for supplying a two polarity switched or alternating current drive signal to produce current flow through the sender which is indicative of fuel level. The presently preferred embodiment uses a 50% duty cycle switched drive signal of alternating positive and negative polarities at a frequency in the range of 100 Hz to 15 kHz. Higher switching frequencies are also possible.
Although the precise electrochemical phenomenon taking place is not fully understood, one explanation is that the use of sufficiently high frequency alternating polarity current prevents the electrically driven oxidation reaction from building up a deposit of silver oxide on the contacts and wiper. It is believed that the electrode oxidation tending to occur during the first polarity phase is reversed or inhibited during the second polarity phase. By effecting polarity reversals at a sufficiently rapid rate, oxidation is not permitted to build up on the electrodes and wiper. Although the invention appears to work over a wide frequency range, some permanent oxidation has been found to occur when polarity is switched every 20 minutes. Accordingly, we prefer to switch polarity at frequencies above 100 Hz, where little or no oxidation has been observed. Frequencies above about 15 kHz are believed to be usable as well, although in vehicular applications such frequencies can be a source of electromagnetic compatibility problems (electromagnetic interference and radio frequency interference).
In addition to being beneficial for vehicular fuel level sensing systems, the switched polarity energizing technique of the invention is more broadly applicable to energizing any immersed electrical component in a vehicular fluid system. Accordingly, the invention also comprises a method for energizing an immersed electrical component whereby a switched polarity signal is generated and delivered to the immersed electrical component as the means for delivering energy to that component. The switched polarity signal periodically alternates between a first polarity and a second polarity as more fully described herein.
For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings.